This is an ongoing study of the role of free radicals in carcinogenesis and the use of ESR-spin-labelling techniques in the study of carcinogen interactions with cellular constituents. The thrust of the free radical studies is in the relationship between free radical induced lipid peroxidation and carcinogenesis. A model lipid peroxidation scheme is used in studying the activation of carcinogens through a free radical process. Spin-label techniques are being applied in the study of carcinogen binding to DNA-chromatin and cellular membranes. Receptor sites are being probed in a similar manner.